Difluorinated Oligothiophenes for High-Efficiency All-Small-Molecule Organic Solar Cells: Positional Isomeric Effect of Fluorine Substitution on Performance Variations

Three symmetrically difluorinated organic semiconductors (namely D5T2F-P, D5T2F-S, and D5T2F-T) containing rhodanine-flanked pentathiophene structures are synthesized and used as donors in all-small-molecule organic solar cells (ASM-OSCs) prepared with the small-molecule acceptor 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC-4F). The different substitutional positions of fluorine atoms (–F) in the conjugated backbone of the donor molecule lead to various material and photovoltaic properties being exhibited. Among the three isomers, the centrally fluorinated D5T2F-P exhibits a redshifted absorption spectrum, downshifted highest occupied molecular orbital (HOMO) energy level, and improved miscibility with IDIC-4F in the blend films, all of which result in superior device performance. The power conversion efficiency (PCE) of the ASM-OSCs based on D5T2F-P:IDIC-4F reaches an impressive value of 9.36% with an open-circuit voltage (V_OC) value of 0.86 V and a short-circuit current density (J_SC) value of 16.94 mA cm⁻², whereas those of D5T2F-S (6.11%) and D5T2F-T (5.42%) are much lower. In comparison, an ASM-OSC based on the nonfluorinated analogue DRCN5T fabricated under the same conditions exhibits poorer performance (8.03% with IDIC-4F), revealing 16% enhancement in the PCE achieved through backbone fluorination. The PCE of 9.36% may be one of the highest efficiencies of oligothiophene-based ASM-OSCs reported in the literature to date.